Conventionally, a NOx reduction system that is capable of efficiently reducing NOx (nitrogen oxide) in exhaust gas discharged from a vehicle and the like has been considered. Most importantly, in diesel cars, the reduction of NOx has been a major concern, as well as the reduction of PM (particulate matter).
Normally, in diesel cars, a diesel oxidation catalyst (DOC) and a diesel particulate filter (DPF) are arranged in the exhaust passage of the diesel engine. However, a combination of the conventional DOC and DPF alone will be insufficient if a further reduction of NOx is required in the future.
As such a method for further reducing NOx in the exhaust gas, for example, in Patent Document 1, an exhaust gas purification device for an internal combustion engine using a NOx absorbent. The NOx absorbent absorbs NOx while the air-fuel ratio of the exhaust gas is lean and releases the absorbed NOx when the oxygen concentration of the exhaust gas is lowered. Thus, the exhaust purification device releases the NOx absorbed while the air-fuel ratio of the exhaust gas is lean from the NOx absorbent when the oxygen concentration of the exhaust gas is lowered.
[Patent Document 1] Japanese Patent No. 2600492